1. Field of the Invention
This invention relates to electromagnetic field activated distance measuring equipment and more particularly to apparatus utilizing a magnetic field to measure the distance to a magnetically permeable object despite the presence of adjacent or intervening magnetically permeable materials, and to measure the depth of a layer of magnetically permeable bulk material.
2. Prior Art
Prior art electromagnetic distance measuring devices have for the most part been of the capacitance or eddy current type. The capacitance type determines the distance to an electrically conductive object by measuring the electrical capacitance between a charged plate and the object. The eddy current type of detector operates on the principle that the magnetic field generated by an alternating electric current, induces eddy currents in electrically conductive objects which in turn generate their own magnetic fields tending to oppose the primary field. By detecting and measuring the effect of these eddy currents, the distance to the object may be determined. An example of such a proximity gauge is disclosed in U.S. Pat. No. 3,609,527 wherein a differential probe is connected in a bridge circuit to detect the effects of the induced eddy currents. Eddy current detectors operate at high frequencies, generally above 50 kilohertz.
Magnetic field sensors have been utilized in detecting the presence of, as opposed to measuring the distance to, magnetically permeable objects. One such system disclosed in U.S. Pat. No. 3,644,825 indicates, in addition to the presence of a metallic object, the direction of movement of the object or if the direction of movement is known, the relative position of the object. It does not, however, indicate the distance to the metallic object.
Devices have also been designed for detecting the presence of metallic objects in metallic or nonmetallic ores. Examples of such devices are illustrated in U.S. Pat. Nos. 2,819,447, 3,588,685 and 3,588,686. All of the devices utilize the eddy current principle to detect electrically conductive tramp metal in ores transported by a conveyor. The first utilizes a high frequency oscillator in which an inductor which comprises one of the components of the oscillator circuit surrounds the ore carrying conveyor belt. The passage of tramp metal through the window in the inductor alters its effective inpedance and thereby changes the oscillator frequency. U.S. Pat. No. 3,588,686 operates in a similar manner but at a frequency of 5 to 20 kHz and provides a clamping circuit to prevent tripping of the alarm signal as the metal lacings in the belt pass through the inductor coil. In U.S. Pat. No. 3,588,685 the phase shift caused by eddy currents induced in the tramp metal is detected by an antenna system in which the perpendicular projection of the antennae on the plane of the conveyor is trapezoidal such that the foreign body is detected irrespective of its shape, orientation or position on the conveyor. All of the devices detect only the presence and not the distance to the detected object.
It is an object of the present invention to provide an economical, reliable, noncontact means of determining distance to an object whether or not there is intervening material. More particularly, the invention is directed to determining the distance to a large metal object such as a steel plate and especially in the presence of intervening bulk material which may or may not be magnetically permeable. The invention is also useful in determining the depth of magnetically permeable bulk material resting on a planar, nonmagnetic surface. It is an object of the invention to utilize a magnetic field and the magnetic properties of the object and any intervening material to effect the measurement.